rbd: get image features for a v2 image

The features values for an rbd format 2 image are fetched from the
server using a "get_features" method.  The same method is used for
getting the features for a snapshot, so structure this addition with
a generic helper routine that can get this information for either.

The server will provide two 64-bit feature masks, one representing
the features potentially in use for this image (or its snapshot),
and one representing features that must be supported by the client
in order to work with the image.

For the time being, neither of these is really used so we keep
things simple and just record the first feature vector.  Once we
start using these feature masks, what we record and what we expose
to the user will most likely change.

Signed-off-by: Alex Elder <elder@inktank.com>